Mechanical resonating structures including a temperature compensation structure

ABSTRACT

Mechanical resonating structures are described, as well as related devices and methods. The mechanical resonating structures may have a compensating structure for compensating temperature variations.

RELATED APPLICATIONS

The present application claims the benefit of U.S. Provisional PatentApplication Ser. No. 61/138,171, filed Dec. 17, 2008 and titled“Mechanical Resonating Structures Including a Temperature CompensationStructure,” which application is hereby incorporated by reference hereinin its entirety.

FIELD OF INVENTION

The invention relates generally to mechanical resonating structures, andmore particularly, to mechanical resonating structures having atemperature compensation structure, as well as related devices andmethods.

BACKGROUND OF INVENTION

Mechanical resonators are physical structures that are designed tovibrate at high frequencies. Such resonators may be incorporated into avariety of devices such as timing oscillators, mass sensors, gyros,accelerometers, switches, and electromagnetic fuel sensors, amongstothers.

During use, mechanical resonators, and the devices which incorporate thesame, may be exposed to different temperature conditions and variations.Such conditions and variations can cause material expansion andcontraction, as well as changes in material stiffness. This can resultin a variation in vibrational characteristics (e.g., resonatingfrequency) across the temperature range. These effects also can lead toincreased noise, reduction in bandwidth, deterioration of signal qualityand can, in general, create stability problems in devices.

The temperature stability of a mechanical resonator may be quantified asthe temperature coefficient of frequency (TCF), which is expressed as:TCF=(1/f)(∂f/∂T), where f is the resonance frequency and T is thetemperature. Another term that is used to quantify the stiffnesscomponent of the temperature stability of a mechanical resonator (whichis one of the primary contributors to TCF) is the temperaturecoefficient of stiffness (TCS), which can be expressed as:TCS=(1/C_(eff))(∂C_(eff)/∂T), where C_(eff) is the effective stiffnesscoefficient of the resonator.

To address the effects resulting from temperature change, it can beadvantageous for mechanical resonating structures to have temperaturecompensation capabilities to improve the stability of such structures,and associated devices, over a range of temperatures.

SUMMARY OF INVENTION

Mechanical resonating structures, as well as related devices andmethods, are described herein.

In one aspect, a device is provided comprising a mechanical resonatingstructure. The mechanical resonating structure includes an active layerand a compensating structure coupled to the active layer. Thecompensating structure comprises a first layer having a stiffness thatincreases with increasing temperature over at least a first temperaturerange, a third layer having a stiffness that increases with increasingtemperature over at least the first temperature range, and a secondlayer between the first layer and the third layer.

According to another aspect, a device is provided comprising amechanical resonating structure. The mechanical resonating structurecomprises an active layer and a compensation structure coupled to theactive layer and configured to compensate temperature-induced variationsin stiffness of at least the active layer. The compensation structurecomprises a first layer, a second layer, and a third layer. The firstand third layers are formed of a first material and the second layer isformed of a second material different than the first material. Thesecond layer is disposed between the first layer and the second layer.

This Summary is not exhaustive of the scope of the various aspects ofthe present invention described herein. Moreover, this Summary is notintended to be limiting of the various aspects and should not beinterpreted in that manner. While certain embodiments have beendescribed and/or outlined in this Summary, it should be understood thatthe various aspects are not limited to such embodiments, descriptionand/or outline, nor are the claims limited in such a manner. Indeed,many others embodiments, which may be different from and/or similar to,the embodiments presented in this Summary, will be apparent from thedescription, illustrations and claims which follow. In addition,although various features, attributes and advantages have been describedin this Summary and/or are apparent in light thereof, it should beunderstood that such features, attributes and advantages are notrequired whether in one, some or all of the embodiments and, indeed,need not be present in any of the embodiments of the various aspects.

Other aspects, embodiments and features of the invention will becomeapparent from the following detailed description of the invention whenconsidered in conjunction with the accompanying drawings. All patentapplications and patents incorporated herein by reference areincorporated by reference in their entirety. In case of conflict, thepresent specification, including definitions, will control.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a 3-D top view of a mechanical resonating structureaccording to embodiments of the present invention.

FIG. 2A shows a cross-sectional view of a mechanical resonatingstructure according to certain embodiments of the present invention.

FIG. 2B shows a cross-sectional view of a mechanical resonatingstructure according to certain embodiments of the present invention.

FIG. 3A shows a diagram of an uncompensated mechanical resonatingstructure with a negatively-sloped temperature coefficient of frequency.

FIG. 3B shows a diagram of a mechanical resonating structure with anegatively-sloped temperature coefficient of frequency according toembodiments of the present invention.

FIG. 3C shows a diagram of a mechanical resonating structure with anapproximately zero temperature coefficient of frequency according toembodiments of the present invention.

FIG. 3D shows a diagram of a mechanical resonating structure with apositively-sloped temperature coefficient of frequency according toembodiments of the present invention.

FIG. 3E shows a graph of the normalized frequency variation (Δf/f)versus temperature of the mechanical resonating structures in FIGS.3A-3D as a function of temperature according to embodiments of thepresent invention.

FIG. 3F shows a graph of a mechanical resonating structures' non-lineartemperature coefficient of frequency behavior according to embodimentsof the present invention.

FIG. 3G shows a graph of a mechanical resonating structures' non-lineartemperature coefficient of frequency behavior where all the structureshave a zero temperature coefficient of frequency at room temperatureaccording to embodiments of the present invention.

FIGS. 3H-3J show diagrams of mechanical resonating structures withdifferent layer thicknesses having a zero temperature coefficient offrequency at room temperature and varying non-linear temperaturecoefficients of frequency according to embodiments of the presentinvention.

FIG. 4A shows a diagram of a planarized configuration of a mechanicalresonating structure according to embodiments of the present invention.

FIG. 4B shows a diagram of a non-planarized configuration of amechanical resonating structure according to embodiments of the presentinvention.

FIG. 4C shows a diagram of a configuration of a mechanical resonatingstructure according to embodiments of the present invention.

FIG. 5A shows a diagram of a planarized configuration of a mechanicalresonating structure with top and bottom electrodes according toembodiments of the present invention.

FIG. 5B shows a diagram of a non-planarized configuration of amechanical resonating structure with top and bottom electrodes accordingto embodiments of the present invention.

FIG. 5C shows a diagram of a configuration of a mechanical resonatingstructure with top and bottom electrodes according to embodiments of thepresent invention.

FIGS. 6A-6G illustrate steps for fabricating a mechanical resonatingstructure using a first method according to embodiments of the presentinvention.

FIGS. 7A-7F illustrate steps for fabricating a mechanical resonatingstructure using a second method according to embodiments of the presentinvention.

FIGS. 8A-8B show configurations of a mechanical resonating structurethat suppress spurious frequencies according to embodiments of thepresent invention.

FIGS. 9A-9B illustrate a two-port mechanical resonating structureaccording to embodiments of the present invention.

FIGS. 10A-10B illustrate a four-port mechanical resonating structureaccording to embodiments of the present invention.

In the drawings, the same reference numbers identify identical orsubstantially similar elements or acts. The drawings illustrateparticular embodiments for the purpose of describing the claimedinvention, and are not intended to be exclusive or limiting in any way.The figures are schematic and are not intended to be drawn to scale. Inthe figures, each identical, or substantially similar component that isillustrated in various figures is represented by a single numeral ornotation. For purposes of clarity, not every component is labeled inevery figure. Nor is every component of each embodiment of the inventionshown where illustration is not necessary to allow those of ordinaryskill in the art to understand the invention.

In the course of the detailed description to follow, reference will bemade to the attached drawings. These drawings show different aspects ofthe present invention and, where appropriate, reference numeralsillustrating like structures, components, materials and/or elements indifferent figures are labeled similarly. It should be understood thatvarious combinations of the structures, components, materials and/orelements, other than those specifically shown, are contemplated and arewithin the scope of the present inventions.

DETAILED DESCRIPTION

Mechanical resonating structures, as well as related devices andmethods, are described herein. The mechanical resonating structuresinclude an active layer comprising an active material (e.g., apiezoelectric material). For example, the active layer may be formed ofa piezoelectric material. The stiffness of the active layer generallyvaries across the range of temperature to which the mechanicalresonating structures are exposed during use. As described furtherbelow, the mechanical resonating structures include a compensatingstructure that can be designed to have a stiffness variation withtemperature such that it balances the stiffness variation withtemperature of the active layer and/or any additional layers of themechanical resonating structure (e.g., electrode layers, support layers,or any other layers of the mechanical resonating structure) to give themechanical resonating structure a targeted stiffness variation over thetemperature range. According to one aspect, the compensating structurecan be designed such that the mechanical resonating structure has adesired frequency variation with temperature, for example bycompensating for any one or more (including all) of the following:temperature-induced variations in stiffness of the materials of themechanical resonating structure; temperature-induced expansion and/orcontraction of materials; stresses caused by different coefficients ofthermal expansion of different materials of the mechanical resonatingstructure; interfacial stresses arising from interfaces betweenmaterials of the mechanical resonating structure; stresses generated bya substrate and/or anchors connected to the mechanical resonatingstructure (in those embodiments in which the mechanical resonatingstructure is coupled to a substrate by anchors); and stresses arisingfrom packaging of the mechanical resonating structure. For example, thecompensating structure can be designed so that the resonant frequency ofthe mechanical resonating structure does not vary much, or at all, overa wide temperature range (e.g., TCF approaches, or is equal to 0). Thecompensation, thus, can significantly reduce undesirable effects thatwould result from such variation including a deterioration in signalquality and stability, amongst others.

FIG. 1 shows a mechanical resonating structure 100 according to anembodiment. The mechanical resonating structure is connected to pads 102via anchors 104 according to this embodiment. As described furtherbelow, the mechanical resonating structure vibrates in response to asource of excitation (e.g., application of an electrical potential), andin some embodiments is configured (shaped, sized, etc.) to support oneor more modes of Lamb waves. The mechanical vibration of the mechanicalresonating structure may be converted to an electrical output signalwhich, for example, may be further processed. The mechanical resonatingstructure can generate signals with multiple modes and resonantfrequencies, and, as mentioned, in some embodiments may be configured tosupport one or more modes of Lamb waves, although not all embodimentsare limited in this respect. Typically, one of the modes can dominateand the mechanical resonating structure can vibrate at the resonantfrequency associated with the dominant mode. The mechanical resonatingstructure can include a resonating structure plate 106 and interdigitaltransducer (IDT) electrodes 202. The mechanical resonating structure caninclude an active layer 204, as described further below.

The frequency produced by the mechanical resonating structure may varydepending on the design and application. For example, the frequencyproduced may be between a 1 kHz and 10 GHz. In some embodiments, forexample, the frequencies are in the upper MHz range (e.g., greater than100 MHz), or at least 1 GHz (e.g., between 1 GHz and 10 GHz). In somecases, the signal may have a frequency of at least 1 MHz (e.g., 13 MHz,26 MHz) or, in some cases, at least 32 kHz.

The dimensions of the mechanical resonating structure depend, in part,on the desired performance including the desired frequency. According tosome embodiments, the mechanical resonating structure can be amicromechanical resonator. The mechanical resonating structure may havea “large dimension” (i.e., the largest of the length, width, thickness,etc.) of less than 1 mm; in some cases, the large dimension is less than500 micron, or less than 100 micron, or less than 10 micron.

The mechanical resonating structure may have any suitable shape. Forexample, the configuration of the mechanical resonating structure caninclude, for example, any antenna type geometry, as well as beams,cantilevers, free-free bridges, free-clamped bridges, clamped-clampedbridges, discs, rings, prisms, cylinders, tubes, spheres, shells,springs, polygons, diaphragms and tori. Any of the mechanical resonatingstructure elements may be formed either in whole or in part of the sameor different geometries. In addition, several different type geometricalstructures may be coupled together to obtain particular resonance moderesponses, as described further below. For example, FIG. 8A shows amechanical resonating structure design with an IDT electrodeconfiguration that allows reduction in coupling of spurious frequenciesand their associated modes. In another example illustrated in FIG. 8B,additional anchors 804 may be added to support a mechanical resonatingstructure. The anchors can be placed at locations of minimumdisplacement (of the mechanical resonating structure), so that spuriousresonator modes can be suppressed. Similarly, geometrical and structuralalterations can be made to improve quality (e.g., Q-factor, noise) ofthe signal generated by the mechanical resonating structure.

In some embodiments, the mechanical resonating structure may include aplurality of resonating elements. At least some of the resonatingelements may be coupled to one another. In some of these embodiments,the resonating elements may have different dimensions. For example, themechanical resonating structure may include at least one major elementthat has a large dimension that is larger than the large dimension ofthe minor element. In general, the minor elements have at least onesmaller dimension (e.g., length, thickness, width) than the majorelement. Minor elements can have a shorter length than the majorelement. The minor elements may have nanoscale (i.e., less than 1micron) dimensions. In some embodiments, at least one of the dimensionsis less than 1 micron; and, in some embodiments, the large dimension(i.e., the largest of the dimensions) is less than 1 micron.

Suitable mechanical resonating structures have been described, forexample, in International Publication No. WO 2006/083482, U.S. patentapplication Ser. No. 12/181,531, filed Jul. 29, 2008, and in U.S. patentapplication Ser. No. 12/142,254, filed Jun. 19, 2008 and published Oct.1, 2009 as U.S. Patent Application Publication 2009-0243747, which areincorporated herein by reference in their entireties. It should beunderstood that a number of different designs for the mechanicalresonating structure are also suitable.

FIG. 1 also shows one configuration of IDT electrodes and the resonatingstructure plate according to some embodiments. Other suitableconfigurations of electrodes can be employed as shall be discussed infurther detail below.

FIG. 2A illustrates a lateral view of a mechanical resonating structureaccording to some embodiments. The mechanical resonating structure canbe built using several components, layers, and materials including IDTelectrodes 202, active layer 204, electrode layer(s) 206 and acompensating structure 208.

The active layer 204 responds to the transduction method used to actuatethe mechanical resonating structure (i.e., cause to vibrate) and/ordetect motion of the mechanical resonating structure. It should beunderstood that any transduction method may be used includingpiezoelectric, piezoresistive, electrostatic, electrostrictive,electromotive, magnetostrictive, magnetomotive, thermal, spin-torqueeffect, and spin-polarized current driven magnetic excitation, amongstothers.

The active layer may have any suitable construction (includingcomposition) which will depend, in part, on the transduction method usedfor actuation and/or detection. In some embodiments, the active layer isformed of a piezoelectric material. In some embodiments, the activelayer is formed of a semiconductor material such as silicon. It shouldbe understood that other compositions are also possible. In some cases,the active layer is formed of multiple layers. For example, the activelayer may comprise multiple layers, one or more of which are functional(e.g., piezoelectric) and one or more of which are not.

As noted above, the active layer may be formed of a piezoelectricmaterial. Examples of suitable materials include aluminum nitride (AlN),zinc oxide (ZnO), cadmium sulfide (CdS), quartz, lead titanate (PbTiO₃),lead zirconate titanate (PZT), lithium niobate (LiNbO₃), and lithiumtantalate (LiTaO₃). In some embodiments, AN may be preferred. Mostactive layer materials (e.g., silicon, piezoelectric materials) normallyhave a negative temperature coefficient of stiffness (TCS). That is,most active layer materials may become less stiff (also referred to as“softer”) as temperature increases over a range. Stiffness, in general,can be associated with a resistance of a material to deform in responseto an applied force.

As mentioned, according to one aspect of the present invention, amechanical resonating structure may comprise a compensation structure,such as the compensation structure 208 of FIG. 2A. The compensationstructure may be configured to provide a desired stiffness variation ofthe mechanical resonating structure and/or frequency of operationvariation of the mechanical resonating structure over a desiredtemperature range (e.g., an anticipated operational temperature range ofthe mechanical resonating structure) for one or more modes of vibrationof interest. In some embodiments, the composition of the active layer ofthe mechanical resonating structure may be considered in configuring thecompensation structure, as the composition of the active layer mayimpact the stiffness variation of the active layer with temperature,which is to be compensated by the compensation structure in someembodiments. According to one embodiment, the compensation structure maybe configured to provide the mechanical resonating structure with a TCFhaving an absolute value of less than approximately 1 ppm/K over atemperature range of at least 40° C. centered around room temperature(25° C.) for one or more modes of Lamb waves when the active layer isformed of aluminum nitride. However, this is merely a non-limitingexample provided for purposes of illustration.

In the illustrated embodiment of FIG. 2A, active layer 204 is formed oncompensation structure 208. Other configurations are also possible. Forexample, in some cases, the compensation structure may be formed on theactive layer

As shown, compensation structure 208 includes multiple components (e.g.,layers). In general, characteristics (e.g., composition, dimensions, andarrangement within the structure) of the components (e.g., layers) areselected such that structure 208 provides the desired compensation withrespect to the active layer and any additional layers to be compensated,so that the mechanical resonating structure exhibits a desired behavioracross a range of temperatures for any modes of vibration of interest.

In the embodiment shown in FIG. 2A, the compensating structure includesa first layer 210 and a second layer 212. The stiffness of layers 210,212 may vary differently with temperature. For example, layer 210 mayhave a stiffness that increases with increasing temperature over atemperature range (i.e., a positive TCS). Layer 212 may have a stiffnessthat decreases, or stays relatively constant, with increasingtemperature over a temperature range (i.e., a negative TCS). Asdescribed further below, the arrangement of the first and second layers(e.g., dimensions, location within structure) may be selected to impartthe mechanical resonating structures with desired behavior across arange of temperatures. For example, the arrangement may be selected sothat the resonating structures have a relatively constant stiffness overa temperature range. That is, the TCS may approach or be equal to 0.This may contribute to minimizing the frequency variation over thetemperature range (e.g., TCF may approach or be equal to 0). Thus, itshould be appreciated that in some embodiments the temperaturecompensation structure may compensate for temperature-induced variationsin stiffness of layers other than the active layer (but in addition tothe active layer in some embodiments) of the mechanical resonatingstructure, e.g., one layer of the temperature compensation structure maycompensate for temperature-induced stiffness variations of another layerof the temperature compensation structure.

It should be understood that, in certain embodiments, the compensatingstructure may include one or more layers in addition to those shown inFIG. 2A. Some of these embodiments are described further below. Theadditional layer(s) may have the same composition as one of the first orsecond layers. In other embodiments, the additional layer(s) may have adifferent compensation than both the first and second layers.

In some embodiments, the compensation structure may be formed of only asingle layer (e.g., first layer 210). In one such embodiment, forexample, the active layer may be formed of silicon and the single layerof the compensation structure may be formed of SiO₂. In an alternativesuch embodiment, the active layer may be formed of aluminum nitride(AlN) and the single layer of the compensation structure may be formedof silicon dioxide (SiO₂). Other choices for the materials may also beused.

The first layer can have characteristics that are selected so that ithas a positive TCS (i.e., TCS>0) over a temperature range. For example,the composition of the first layer may be selected to provide a positiveTCS. Suitable compositions can include SiO₂ and Al₂O₃, amongst others.In some cases, SiO₂ may be preferred. In some cases, the first layer maybe composed of a series of ultra-thin layers (e.g., less than 10 nmthick) which are combined to produce an overall layer having a positiveTCS. The positive TCS may also, or alternatively, be engineered byimplanting species (e.g., ions, neutrons) into the first layer. Thus, itshould be understood that a layer exhibiting a positive TCS may beobtained in any of a number of suitable ways, and that the variousaspects described herein including one or more layers exhibiting apositive TCS are not limited in the manner in which the positive TCS isobtained.

As noted above, first layer 210 can have a positive TCS over atemperature range. In some cases, the TCS is positive across the entireoperating temperature range of the device. For example, the TCS may bepositive across the temperature range of between −55° C. and 150° C., orbetween −40° C. and 85° C. However, in other cases, the TCS of firstlayer 210 may be positive across a portion of the operating range, andnegative across other portion(s). The TCS of the first layer may bepositive across the majority of the temperature range. In someembodiments, the TCS of the first layer may be positive across a rangeof at least 200° C.; in some embodiments, at least 100° C.; and, inother embodiments, at least 50° C.

As noted above, second layer 212 may have a differentstiffness-temperature dependence than the first layer. The second layermay be a support layer that provides robustness to the first layer. Thesecond layer may be formed of a material having a lower acoustical lossthan the material of the first layer. In some embodiments, the secondlayer is formed of a material having a certain crystal structure. Forexample, the second layer may be formed of a single crystal materialand/or a material having higher crystal quality than the material of thefirst layer (e.g., lower defects). In particular, when the first layercomprises SiO₂, the robustness and support provided by the second layeris useful, since a structure comprised of a thin SiO₂ layer(s) and theactive layer can be fragile and prone to damage if subjected to forcefulmechanical movements or vibrations. The second layer can also provideimproved signal performance (e.g., less noise and better Q-factor).Suitable materials for second layer 212 include silicon, diamond,silicon carbide, sapphire, quartz, germanium, aluminum nitride, andgallium arsenide, amongst others. In some embodiments, it is preferablefor the second layer to be formed of silicon.

The embodiment of FIG. 2A includes IDT electrodes 202 and an electrodelayer 206 to facilitate transfer of charges and electric potentialacross a mechanical resonating structure. The number of electrodes andplacement of electrodes can be important as they can determine the typesof acoustic waves and excitation modes generated by the mechanicalresonating structure's motion.

Examples of suitable electrode materials include, but are not limitedto, aluminum (Al), molybdenum (Mo), titanium (Ti), chromium (Cr),ruthenium (Ru), gold (Au), platinum (Pt) or AlSiCu. In general, anysuitable electrode material can be utilized for the electrode layer. Insome embodiments, a thin layer of Ti and/or AN may be added beneath theelectrode to enhance crystal orientation of the active (e.g.,piezoelectric) material layer.

FIG. 2B illustrates another embodiment in which the compensatingstructure includes a third layer 214. In some cases, the third layer maybe formed of a material having a positive TCS. Suitable materials havinga positive TCS were described above in connection with first layer 210.In some embodiments, the third layer comprises the same material asfirst layer 210. However, in other embodiments, the third layer maycomprise a different material than the first layer (and the secondlayer). In some embodiments, layers 210 and 214 are formed of SiO₂layers. In some of these cases, the second layer 212 is formed of Si. Asshown, the second layer is positioned between the first layer and thethird layer. Other arrangements are possible.

In some embodiments, the third layer has a similar thickness as thefirst layer. For example, the ratio of the thickness of the third layerto the thickness of the first layer may be between 1:0.25 and 1:4.0,between 1:0.75 and 1:1.25, or about 1:1. For example, the listed ratiosmay be suitable when the third layer is formed of the same compositionas the first layer (e.g., when the first and third layers are bothformed of SiO₂, or any other suitable material (e.g., any other suitablepositive TCS material)).

In some cases, the three-layer compensation structure configurationillustrated in FIG. 2B may provide enhanced performance as compared to atwo-layer compensating structure. For example, such a configuration canreduce the tendency of the resonating structure to bend out of plane bybalancing residual stress in the structure. This can provide a high Q,low noise signal. In some embodiments, a split-layer compensationstructure similar to that illustrated in FIG. 2B may facilitatefabrication of the structure. For example, as mentioned, in oneembodiment the layers 210 and 214 may be formed of the same material(e.g., SiO₂). Rather than forming a single layer of the material havinga thickness approximately equal to the combined thickness of layers 210and 214, the configuration of FIG. 2B may be used in which separatelayers 210 and 214 are formed. In this manner, fabrication defectsassociated with forming thick material layers (e.g., cracking, bending,warping, etc.) may be minimized or avoided, as may be out-of-planedeformation of the mechanical resonating structure.

It should be understood that the compensation structure may have avariety of different configurations in addition to those shown in FIGS.2A and 2B. For example, the compensation structure may include more thanthree layers. In some cases, the compensation structure may include atleast one additional layer having a similar function as second layer 212described above. The additional one or more layer(s) may be formed of amaterial having a lower acoustical loss than the material of the firstlayer including those noted above. In some cases, the additional one ormore layer(s) is formed of silicon. As noted above, the compensationstructures can be designed to provide the mechanical resonatingstructure with a desired frequency variation with temperature (e.g.,TCF) for one or more modes of interest. In some embodiments, it may bedesirable for the TCF to approximate or be equal to zero over a range oftemperatures for one or more modes of Lamb waves, or for any other modesof interest. That is, in these cases, the compensating structure canenable the mechanical resonating structure to operate with little or novariation in frequency over a range of temperatures.

However, in some embodiments, it may be desirable for the TCF to benon-zero at least over certain temperature ranges. Thus, in these cases,the frequency of the mechanical resonating structure may vary a desiredamount with temperature. In these embodiments, the compensationstructure is designed to achieve the desired amount of variation.

In some embodiments, the mechanical resonating structure has an absolutevalue of a TCF of less than 10 ppm/K over a range of temperatures. Forexample, the absolute value of the TCF may be less than 10 over ananticipated operating range of the mechanical resonating structure(e.g., from −40° C. to 85° C.). In some embodiments, the absolute valueof the TCF is less than 6 ppm/K over a range of temperatures, forexample from −40° C. to 85° C. In some embodiments, the absolute valueof the TCF over the range of temperatures (e.g., from −40° C. to 85° C.)is less than 5 ppm/K, or less than 3 ppm/K, less than 2 ppm/K or lessthan 1 ppm/K. In some cases, the TCF may approximately equal 0 (whichincludes exactly equaling zero) over a range of at least 5° C. or atleast 10° C. within the range from −40° C. to 85° C., as a non-limitingexample. Other values are also possible. For example, in someembodiments the absolute value of the TCF may be less than 4 ppm/K, lessthan 1 ppm/K, less than 0.5 ppm/k, or approximately zero, over a rangeof temperature spanning at least 40° C. (e.g., a range of temperaturesspanning at least 40° C. and centered approximately at room temperature,25° C.).

The range of temperatures over which the desired TCF is achieved maydepend on the application. In some cases, the temperature range may bebroad. For example, the temperature range may be between −55° C. and150° C.; or, −40° C. to 85° C. The range may span at least 200° C., atleast 100° C., at least 75° C., at least 50° C., or at least 40° C. Inother embodiments, the range of temperature over which the desired TCFis achieved may be more narrow. For example, the temperature range maybe less than 50° C., less than 25° C., or less than 10° C. In general,the above-noted ranges of temperatures can be centered around anydesired temperature. For example, they may be centered around roomtemperature (i.e., 25° C.), an elevated temperature such as 100° C., orotherwise.

The compensation structure may be designed to result in a mechanicalresonating structure with a desired TCF by selecting appropriatecharacteristics for the compensation structure. For example, thecharacteristics may include the composition, dimensions, and arrangementof layers within the structure.

In some embodiments, there may be a desired thickness ratio betweenlayers in the structure. In some cases, the thickness of the activelayer (e.g., layer 204) and the total thickness of the positive TCSmaterial layer(s) (e.g., layer 210 in FIG. 2A and layers 210, 214 inFIG. 2B) may be selected to provide a desired ratio. The ratio of thethickness of the active layer to the total thickness of the positive TCSmaterial layer(s) may be between 1:1 and 1:500, or between 1:1 and 1:200in some non-limiting embodiments. In some embodiments, the ratio may bebetween 1:1 and 1:10, or between 1:4 and 1:8, or between 1:5 and 1:7(e.g., about 1:6). In some such embodiments, the active layer may beformed of aluminum nitride and the positive TCS material of thecompensation structure may be formed of silicon dioxide, although othermaterials may be used for the active layer and positive TCS materiallayer, as those listed are merely examples. In embodiments which includemore than one layer formed of a positive TCS material, the totalthickness of the positive TCS material layer(s) includes the sum of thethickness of all such layers. In embodiments which include a singlelayer formed of a positive TCS material, the total thickness of thepositive TCS material layer(s) is the thickness of that single layer.The above-noted ratios, for example, may be suitable when the positiveTCS material in the layer(s) is SiO₂ and the active material is apiezoelectric material such as AlN. Other ratios may be suitabledepending on the materials used.

In some cases, the thickness of the positive TCS material layer(s) andthe thickness of the layer(s) having a lower acoustic loss than thepositive TCS material layer(s) (e.g., layer 212) are selected to providea desired ratio. For example, the ratio of the total thickness of thepositive TCS material layer(s) (e.g., the combined thickness of multiplepositive TCS layers in those embodiments in which the compensationstructure includes multiple positive TCS layers) and the layer(s) havinga lower acoustic loss than the positive TCS material layer(s), may bebetween 1:0.1 and 1:10, 1:0.5 and 1:3, between 1:0.75 and 1:1.25, orbetween 1:1 and 1:2. The above-noted ratios may be suitable when, forexample, the positive TCS material in the layer(s) is SiO₂ and thelayer(s) having a lower acoustic loss than the positive TCS materiallayer(s) is/are formed of Si. These ratios may be suitable when theactive material is a piezoelectric material such as AlN.

According to some embodiments, the ratio of the thickness of the activelayer(s) of the mechanical resonating structure compared to thethickness of any layers of the temperature compensation structure havinglower acoustic loss (e.g., layer 212) may be designed to fall withincertain ranges. For example, according to one embodiment the ratio ofthe thickness of the active layer to the total thickness of one or morelayers of the temperature compensation structure having lower acousticloss than the positive TCS layer(s) may be between 1:0.1 and 1:500, andin some embodiments may be between 1:0.5 and 1:20. Such ratios may besuitable when the active layer comprises, for example, AlN, and thelayer of lower acoustic loss material comprises, for example, silicon.Other materials and other ratios may be used, however, as those listedare not limiting.

It should be understood that certain embodiments may include layerthicknesses outside of the above-noted ranges.

FIGS. 2-3D and FIGS. 3H-3J illustrate some embodiments of the invention.Other configurations of a mechanical resonating structure can beutilized. For example, FIGS. 4A-4C illustrate 3 different configurationsof a mechanical resonating structure. As shown in FIG. 4A, a topelectrode 202A can be placed on top of active layer 204 (e.g., AlN). Thecompensating structure can be situated below the active layer with IDTelectrodes 202 located at an interface of the compensating structure andthe active layer. The compensating structure can be a Si layer 212placed between two SiO₂ layers 210, 214 of equal thickness. FIG. 4Aillustrates a configuration in which the IDT electrodes are not placedon the top surface of the mechanical resonating structure and in whichthe active layer is planarized. In contrast, FIG. 4B illustrates asimilar structure to FIG. 4A with a difference that the top electrodeand active layer are not planarized. FIG. 4C also illustrates aconfiguration similar to FIG. 4A; however, in contrast to FIG. 4A, theIDT electrodes in FIG. 4C are embedded in a first SiO₂ layer of thecompensating structure.

FIGS. 5A-5C illustrate another set of embodiments in which top 202 andbottom 504 IDT electrodes are used in a mechanical resonating structure.For example, in FIG. 5A, bottom electrodes are placed within the activelayer 204 at an interface with the compensating structure as in FIGS. 4Aand 4B. However, an additional set of top IDT electrodes are depositedon the top surface of the active layer. FIG. 5B illustrates anon-planarized structure as compared to the planarized structured inFIG. 5A. FIG. 5C shows the bottom IDT electrodes being situated withinthe first SiO₂ layer 210 of the compensating structure. As can beappreciated from the descriptions of FIGS. 4A-5C, various configurationsof a mechanical resonating structure with zero TCS can be designed andutilized.

A mechanical resonating structure can be manufactured using simplefabrication processes. As an example, FIGS. 6A-6G and FIGS. 7A-7Fillustrate two possible processes used to fabricate a mechanicalresonating structure according to embodiments of the invention. Itshould be understood that other fabrication techniques are also possibleincluding techniques in which the specific process steps are re-arrangedin a different order.

A first fabrication process is shown in FIGS. 6A-6G. Structure 600A,including a handle layer 602, a first layer 210 with a stiffness thatincreases with temperature (e.g., SiO₂ or any suitable oxide layer), anda second layer 212 with a stiffness that decreases with temperature(e.g., Si), can be used to commence the fabrication process. As shown inFIG. 6A, the first layer can be buried between the handle layer and thesecond layer. In some embodiments, the handle and second layers can beSi layers.

Next, as shown in FIG. 6B, thermal oxide layers 604, 214 can be formedon a top surface of the handle layer and a bottom surface of the secondlayer using a suitable thermal oxidation procedure. The added thermaloxide layers can be similar to a thickness of the buried first layer210. Subsequently, a wafer 608 with cavity 606 can be bonded tostructure 600A, as illustrated in FIG. 6C. Bonding the wafer tostructure 600A yields a modified structure 600B with a pre-definedcavity. Subsequently, as illustrated in FIG. 6D, the handle layer 602and oxide layer 604 can be removed and a planarized top first layer maybe formed using any suitable planarization procedure (e.g., chemicalmechanical planarization (CMP). After the planarization process, abottom electrode layer 206, an active layer 204 and a top electrodelayer 202A can be deposited on the top, planarized surface of firstlayer 210 (FIG. 6E). Suitable deposition techniques include, forexample, chemical vapor deposition (CVD) and physical vapor deposition(PVD). In general, any suitable deposition technique can be used.Portions of the top electrode layer can then be selectively removedusing any suitable photolithography process. The selective removal canresult in formation of IDT electrodes 202 as shown in FIG. 6F. As a nextstep, the active layer, the electrode layer, the first layer, the secondlayer and the oxide layer can be etched until the cavity is reached toyield a suspended resonating structure 100 as shown in FIG. 6G. Anchorsand other components (e.g., pads, vias) compensating the suspendedresonating structure are not shown in FIG. 6G.

FIGS. 7A-7F illustrate another process that can be used to fabricate amechanical resonating structure according to some embodiments. Like thefirst process, a structure 700A with a handle layer 702, an oxide layer214 and a second layer 212 with a stiffness that decreases withtemperature (e.g., Si) can be used to commence the fabrication process.A first layer 210 with a stiffness that increases with temperature(e.g., SiO₂ or any suitable oxide layer), a bottom electrode layer 206,a active layer 204 and a top electrode layer 202A can be deposited onthe second layer as shown in FIGS. 7B and 7C. The first layer 210 canhave the same thickness as oxide layer 214. Examples of suitabledeposition techniques have been described above. Subsequently, the topelectrode layer is partially removed to form IDT electrodes 202 asdescribed above and shown in FIG. 7D. Using a bottom-up dry or wet etchprocess, the handle layer is selectively etched to the bottom surface ofoxide layer 214 to form a cavity 606 as shown in FIG. 7E. Furtheretching 704 of the active layer, the bottom electrode layer, the secondlayer, the first layer and the oxide layer can result in a suspendedresonating structure 100 illustrated in FIG. 7F.

It should be understood that other configurations and/or fabricationprocesses can be used for a mechanical resonating structure.

The mechanical resonating structures described herein can beincorporated into a variety of devices. According to some embodiments, amechanical resonating structure can be integrated in tunable meters,mass sensors, gyros, accelerometers, switches, and electromagnetic fuelsensors. According to some embodiments, the mechanical resonatingstructure can be integrated in a timing oscillator. Timing oscillatorscan be used in several devices including digital clocks, radios,computers, oscilloscopes, signal generators, and cell phones. Timingoscillators can precisely generate clock signals, for example, as areference frequency to help synchronize other signals that are received,processed, or transmitted by a device in which the timing oscillator isintegrated. In some scenarios, multiple processes are run simultaneouslyon a device and the execution of such processes rely on a clock signalthat can be generated by the mechanical resonating structure. Accordingto some embodiments, a mechanical resonating structure can also becoupled to additional circuitry. For example, additional circuitry mayinclude filters, mixers, dividers, amplifiers or other applicationspecific components and devices.

In some embodiments, the mechanical resonating structure can be used asa multiple port device. For example, as illustrated in FIGS. 9A and 9B,the bottom electrode can be grounded, while the IDT electrodes arecoupled to two ports, namely Port 1 and Port 2. Alternatively, thebottom electrode could be a floating node. In another exampleillustrated in FIGS. 10A and 10B, a dual mechanical resonating structurecan be utilized to create a four-port mechanical resonating structuredevice. In the dual mechanical resonating structure configuration, twomechanical resonating structures can be implemented on the sameresonating structure plate and the ports can be connected to the desiredinputs and outputs.

The following example is provided for illustration purposes and is notintended to be limiting.

EXAMPLE

The following is an example that illustrates that the TCF of amechanical resonating structure can be controlled according to themethods described herein with reference to FIGS. 3A-3J. In this example,layers 210, 214 are formed of SiO₂ and layer 212 is formed of Si.

FIG. 3E is an indicator of how the TCF varies as a function oftemperature. Specifically, FIG. 3E shows a graph of the normalizedfrequency variation (Δf/f) versus temperature. The TCF corresponds tothe slope of this curve. As noted above, the active layer can have anegative TCF, and SiO₂ can have a positive TCF for a specified range oftemperatures.

FIG. 3A shows an example of a mechanical resonating structure with anactive layer (e.g., AlN), a bottom electrode, a Si layer 212 and no SiO₂layers. This structure has a negative TCF of approximately −30 ppm/K(illustrated by line A in FIG. 3E). FIG. 3B shows an example of amechanical resonating structure with a Si layer placed between two SiO₂layers as discussed above. The SiO₂ layers have a relatively smallthickness compared to the Si layer and to corresponding SiO₂ layers inFIGS. 3B and 3C. As illustrated by line B in FIG. 3E, the low thicknesscan result in a TCF that is still negative but greater (i.e., lessnegative) than the TCF of the mechanical resonating structure in FIG.3A. The structure shown in FIG. 3C is similar to the mechanicalresonating structure in FIG. 3B; however, in FIG. 3C, both SiO₂ layershave greater thicknesses. The corresponding line, C, in FIG. 3Eindicates an almost zero TCF for the mechanical resonating structure inFIG. 3C. If the thickness of the SiO₂ layers is further increased, asshown in FIG. 3D, then the TCF of the mechanical resonating structurebecomes non-zero and positive, as shown by D in FIG. 3E. Accordingly, inthese embodiments, the thinner the thickness of the SiO₂ layers, themore negative the TCF of the mechanical resonating structure.

While FIG. 3E illustrates an example of how the normalized frequencyvariation (Δf/f) over temperature can be ‘leveled’ and, therefore, theTCF approaches zero over a range of temperatures by selectingappropriate thicknesses for layers of a mechanical resonating structuredevice, in certain cases it may be more challenging to achieve a flatresponse over a broad range of temperatures (e.g., −40° C. to 85° C.),since many materials have non-linear TCF properties. For example, somematerials may have higher order TCF properties. For such materials, themechanical resonating structure may be designed and/or tuned to providezero TCF properties around the operating temperature (e.g., roomtemperature) or any other desired/pre-determined temperature of themechanical resonating structure. For example, as illustrated in FIG. 3F,a non-linear curve C can have a zero TCF at room temperature (i.e., 25°C.) if suitable thicknesses are chosen for the mechanical resonatingstructure layers. If, for example, a slightly thicker SiO₂ layer isused, a zero TCF can be achieved at a temperature greater than roomtemperature, as indicated by curve C⁺; however, if a slightly thinnerSiO₂ layer is used, a zero TCF can be achieved at a temperature lowerthan room temperature, as indicated by curve C⁻.

The thicknesses of the mechanical resonating structure layers may notonly determine where a zero TCF is achieved in a broad range oftemperatures, but may also help reduce the higher-order nature of themechanical resonating structure layers' non-linear TCF properties. FIG.3G, for example, shows the parabolic TCF profile of three mechanicalresonating structures A, B and C illustrated in FIGS. 3H-3J. Structure Chas a smaller Si/SiO₂ layer thickness than structure B, which has asmaller Si/SiO₂ layer thickness than structure A. Due to differences ina ratio of the thickness of the active material to the thickness of theSi layer and/or the SiO₂ layers, the parabolic profile of all threestructures can be different despite having a zero TCF at roomtemperature. For example, as shown in FIG. 3G, since structure A has thethickest SiO₂ layers, structure A maintains its higher ordercharacteristic with a severely parabolic TCF profile as shown by curveA. In comparison, curve B has a less parabolic TCF profile. Curve Bcorresponds to structure B, which has smaller SiO₂ layer thickness thanstructure A. Similarly, as shown by curve C, structure C has the leastparabolic TCF profile since structure C has the thinnest SiO₂ layers.

It should be understood that the various embodiments shown in theFigures are illustrative representations, and are not necessarily drawnto scale. Reference throughout the specification to “one embodiment” or“an embodiment” or “some embodiments” means that a particular feature,structure, material, or characteristic described in connection with theembodiment(s) is included in at least one embodiment of the presentinvention, but not necessarily in all embodiments. Consequently,appearances of the phrases “in one embodiment,” “in an embodiment,” or“in some embodiments” in various places throughout the Specification arenot necessarily referring to the same embodiment. Furthermore, theparticular features, structures, materials, or characteristics can becombined in any suitable manner in one or more embodiments.

Unless the context clearly requires otherwise, throughout thedisclosure, the words “comprise,” “comprising,” and the like are to beconstrued in an inclusive sense as opposed to an exclusive or exhaustivesense; that is to say, in a sense of “including, but not limited to.”Words using the singular or plural number also include the plural orsingular number respectively. Additionally, the words “herein,”“hereunder,” “above,” “below,” and words of similar import refer to thisapplication as a whole and not to any particular portions of thisapplication. When the word “or” is used in reference to a list of two ormore items, that word covers all of the following interpretations of theword: any of the items in the list; all of the items in the list; andany combination of the items in the list.

Having thus described several embodiments of this invention, it is to beappreciated that various alterations, modifications, and improvementswill readily occur to those skilled in the art. Such alterations,modifications, and improvements are intended to be part of thisdisclosure, and are intended to be within the spirit and scope of theinvention. Accordingly, the foregoing description and drawings are byway of example only.

1. A device comprising: a mechanical resonating structure including: an active layer; and a compensating structure coupled to the active layer, the compensating structure comprising a first layer having a stiffness that increases with increasing temperature over at least a first temperature range, a third layer having a stiffness that increases with increasing temperature over at least the first temperature range, and a second layer between the first layer and the third layer.
 2. The device of claim 1, wherein the active layer comprises a piezoelectric material.
 3. The device of claim 2, wherein the piezoelectric material is aluminum nitride.
 4. The device of claim 1, wherein the compensating structure is configured to provide the mechanical resonating structure with a temperature coefficient of frequency (TCF) having an absolute value of less than 10 ppm/K over a second temperature range from approximately −40° C. to approximately 85° C.
 5. The device of claim 4, wherein the TCF has an absolute value of less than 3 ppm/K over the second temperature range.
 6. The device of claim 4, wherein the TCF has an absolute value of less than 2 ppm/K over the second temperature range.
 7. The device of claim 4, wherein the TCF has an absolute value of less than 1 ppm/K over the second temperature range.
 8. The device of claim 4, wherein the TCF is approximately 0 ppm/K over a range of at least 5° C. within the second temperature range.
 9. The device of claim 1, wherein the compensating structure is configured to provide the mechanical resonating structure with a TCF having an absolute value of less than 4 ppm/K over a second temperature range spanning at least 40° C. and centered approximately at 25° C.
 10. The device of claim 9, wherein the absolute value of the TCF is less than 1 ppm/K.
 11. The device of claim 9, wherein the absolute value of the TCF is less than 0.5 ppm/K.
 12. The device of claim 1, wherein the mechanical resonating structure is configured to support Lamb waves.
 13. The device of claim 12, wherein the active layer comprises aluminum nitride.
 14. The device of claim 1, wherein the first layer and the third layer of the compensating structure are formed of a first material.
 15. The device of claim 1, wherein the first layer and the third layer of the compensating structure have approximately the same thickness as each other.
 16. The device of claim 1, wherein the first layer of the compensating structure is formed of a first material and wherein the second layer is formed of a second material different from the first material.
 17. The device of claim 16, wherein the first material is silicon dioxide and the second material is silicon, and wherein a ratio of a total thickness of one or more layers of the mechanical resonating structure comprising the first material to a total thickness of one or more layers of the mechanical resonating structure comprising the second material is between 1:0.75 and 1:2.
 18. The device of claim 16, wherein a ratio of a total thickness of one or more layers of the mechanical resonating structure comprising the first material to a thickness of the active layer is between 1:0.1 and 1:1.25.
 19. The device of claim 16, wherein a ratio of a total thickness of one or more layers of the mechanical resonating structure comprising the first material to a thickness of the active layer is between 1:0.2 and 1:0.75.
 20. The device of claim 1, wherein the first layer is formed of silicon dioxide.
 21. The device of claim 1, wherein the second layer is formed of a material selected from the group consisting of silicon, silicon carbide, sapphire, quartz, germanium, gallium arsenide, aluminum nitride and diamond.
 22. The device of claim 1, wherein the second layer is formed of silicon.
 23. The device of claim 1, wherein the first temperature range spans at least 100° C.
 24. The device of claim 1, wherein the first temperature range spans at least 200° C.
 25. The device of claim 1, wherein the first temperature range is between −55° C. and 150° C.
 26. The device of claim 1, wherein the first temperature range is between −40° C. and 85° C.
 27. The device of claim 1, wherein the active layer is formed of silicon.
 28. The device of claim 1, wherein the active layer is formed on the compensating structure.
 29. A device comprising: a mechanical resonating structure comprising an active layer and a compensation structure coupled to the active layer and configured to compensate temperature-induced variations in stiffness of at least the active layer, the compensation structure comprising a first layer, a second layer, and a third layer, wherein the first and third layers are formed of a first material and wherein the second layer is formed of a second material different than the first material, and wherein the second layer is disposed between the first layer and the second layer.
 30. The device of claim 29, wherein the active layer is formed of aluminum nitride, the first material is formed of silicon dioxide, and the second material is formed of silicon.
 31. The device of claim 29, wherein the mechanical resonating structure further comprises an electrode layer coupled to the active layer, and wherein the compensation structure is further configured to compensate temperature-induced variations in stiffness of the electrode layer.
 32. The device of claim 31, wherein the first and third layers of the compensation structure are configured to compensate for temperature-induced variations in stiffness of the active layer, the electrode layer, and the second layer.
 33. The device of claim 31, wherein the mechanical resonating structure is configured to support Lamb waves, and wherein the compensation structure is configured to provide the mechanical resonating structure with a temperature coefficient of frequency (TCF) having an absolute value of less than 6 ppm/K over a temperature range spanning at least from approximately −40° C. to approximately 85° C. 